(57) An electrode assembly manufacturing method includes the steps of: assembling an electrode stack; performing a primary heat press operation on the electrode stack while engaging the electrode stack with a gripper; and then performing a secondary heat press operation on the electrode stack while the gripper is disengaged from the electrode stack. The secondary heat press operation may include applying heat and pressure to the electrode stack for a time period from 5 seconds to 60 seconds under a temperature condition from 50°C to 90°C and under a pressure condition from 1 Mpa to 6 Mpa. The step of assembling the electrode stack may include alternately stacking first and second electrodes on an elongated separator sheet, and sequentially folding the separator sheet over a previously-stacked one of the electrodes before a subsequent electrode is stacked. An apparatus for performing the manufacturing method is also disclosed.